PURBECK FORMATIONS. 27 



The fourth tooth (4) preserves the summit of the main cone ; but the base of the crown 

 is mutilated. The main cone is broken away from the fifth tooth, but the two inner 

 cusps, characteristic of Sjjalacotherimn, are instructively shown. The accentuation of the 

 inner, less bulging, surface of the ramus is well shown in the present slab. The thin 

 inner alveolar wall descending sheer from the outlets of the sockets is at first shghtly 

 concave vertically ; then swells out into a convexity toward the lower border ; near which, 

 at the hind part of the fragment, may be discerned, when the specimen is held in a strong 

 light, the termination of the longitudinal (mylo-hyoid) groove (y). 



In the hind portion of jaw (fig. 37 a) the equally characteristic base of the external 

 main cone, with its basal cingulum, is seen in the teeth answering to the sixth and 

 seventh molars. The impression of these before and behind are less intelligibly preserved 

 in the matrix. 



As far as I can safely work away the matrix from the inner side of these teeth, the 

 base (horizontal section) thereto turned seems to be indented ; the working surface 

 terminating on that side in two low cusps, as shown in fig. 36b, answering to those shown 

 in the tooth (5) of fig. 34. 



All the evidence concurs in giving a tricuspid crown to the molar teeth in the 

 present Purbeck fossil, of the character of that of the type-specimens of Spalacotheriiim 

 (figs. 32 — 34), with which the mandibular ramus, so far as it is preserved, corresponds. 



.The last specimen referable to Spalacotherium tricuspidens supplies very satisfactorily 

 the characters of the hind end and rising part of the lower jaw. 



It consists of the left mandibular ramus, wanting the symphysial end, with five more 

 or less mutilated molars in place, the inner side being exposed. It is figured of the 

 natural size in fig. 38, and of twice the natural size at a. This specimen includes the 

 condyle {b) and great part of the coronoid process (c). 



From the inner end of the condyle {b) a ridge curves forward, bounding below the 

 depression, terminating anteriorly in the entry of the dental canal {d). The hinder two 

 thirds of the ridge has been broken away in exposing the specimen, indicative that it has 

 extended inward or transversely to the plane of the rising ramus above, to a greater 

 degree than is here shown. The lower border of the ramus is entire and convex, showing 

 that the angle of the jaw was represented by the inflected ridge, plate or process. 



This marsupial character is well shown in Sarcophihs and Thylacinuss (fig. 5, p. 74) ; 

 but the entry to the dental canal is more advanced in the present extinct marsupial than 

 in those existing species. The condyle is transversely extended, and holds the same 

 relation of level to the alveolar tract as in Thylacinus. A deep emargination divides 

 it from the hind border of the coronoid. The shape of this process resembles that in 

 SarcopMlus. A linear groove is continued from the inner border of the dental foramen 

 to beneath the third molar here in place. 



The most perfect molar — the first in place — shows a fine but well marked ' cingulum ' 



